As a display device with which an act of “reading” can be done without feeling stress, electronic paper display devices called electronic books, electronic newspapers, and the like are being developed. This kind of electronic paper display device is required to be thin, light, hard to be smashed, and low in power consumption. Thus, it is preferable to be formed with a display element with a memory. As a display element used for a display device with a memory, an electrophoretic display element, a cholesteric liquid crystal, and the like are known. Recently, an electrophoretic display element using two or more kinds of charged particles has drawn an attention.
A monochrome display active matrix drive type electrophoretic display device will be described. This electrophoretic display device is constituted by stacking a TFT glass substrate, an electrophoretic display element film, and a counter substrate in this order. TFTs (Thin Film Transistors) that are a great number of switching elements arranged in matrix, pixel electrodes, gate lines, and data lines connected to each of the TFTs are provided on the TFT glass substrate. The electrophoretic display element film is formed by spreading microcapsules of about 40 μm all over in a polymer binder. A solvent is injected inside the microcapsules. In the solvent, two kinds of nanoparticles charged plus and minus, i.e., a white pigment such as oxide titanium particles charged minus and a black pigment such as carbon particles charged plus, are sealed in a dispersedly floated manner. Further, as the counter electrode, a counter electrode giving a reference potential is formed.
The electrophoretic display device operates by moving the white pigment and the black pigment vertically through applying a voltage corresponding to pixel data between a pixel electrode and a counter electrode. That is, when a plus voltage is applied to the pixel electrode, the white pigment charged minus is gathered to the pixel electrode while the black pigment charged plus is gathered to the counter electrode. Thus, provided that the counter electrode side is the display face, black is displayed on the screen (all the examples provided hereinafter will be described while assuming that the counter electrode side is the display face). In the meantime, when a minus voltage is applied to the pixel electrode, the black pigment charged plus is gathered to the pixel electrode while the white pigment charged minus is gathered to the counter electrode. Therefore, white is displayed on the screen.
That is, a plus signal voltage is applied to the pixel electrode when switching the image from white display to black display, a minus signal voltage is applied to the pixel electrode when switching from black display to white display, and 0 (V) is applied when maintaining the current image, i.e., when switching display from white display to white display and from black display to black display. As described, the electrophoretic display device has a memory, so that the signal to be applied is determined by comparing the current image (previous screen) with the next image (updated screen).
While the monochrome display microcapsule type electrophoretic display device has been described above, electrophoretic display devices capable of providing color display without losing a monochrome excellent display performance of the electrophoretic display device close to paper have been developed recently. These devices provide monochrome and color displays by displaying colors of uncharged particles and colors of each of charged particles by using uncharged (or weak charged) particles having no sensitivity in the electric fields and a plurality of particles of same polarity or inverse polarities having sensitivity in the electric field in a solvent (see Patent Documents 1 and 2).
For example, the color electrophoretic display device described in Patent Document 1 is constituted with: a pair of substrates; a solvent sealed in the gap between the pair of substrates; and three different colors (e.g., cyan C, magenta M, and yellow Y) of electrophoretic particles charged plus or minus as well as uncharged white particles (W) contained in the solvent. The threshold voltages of the three different colors of electrophoretic particles when starting migration are different from each other. Thus, it is possible with a single cell to display not only white (W) and black (K) but also cyan (C), magenta (M), yellow (Y) as well as secondary colors and tertiary colors of CMY through applying a voltage by using the difference in each of the threshold voltages.
A driving method for providing color display by using the difference in the threshold voltages through placing the charged particles C, M, Y and the white particles W on the same pixel electrode will be described by referring to FIG. 18. Hereinafter, the threshold voltages of the charged particles C, M, and Y are defined as Vth(c), Vth(m), and Vth(y) and assumed to be in a relation of |Vth(c)|<|Vth(m)|<Vth(y). Further, the applied voltages V1, V2, and V3 are assumed to satisfy relations of |Vth(c)|<|V3|<|Vth(m)|, |Vth(m)|<|V2|<|Vth(y)|, and |Vth(y)|<|V1|.
FIG. 18 is a hysteresis loop of the charged particles C, M, and Y, which shows the relation between the applied voltage (threshold voltage) and the relative color density. In this chart, for simplifying the explanation, the moving time in which the charged particles Y, M, and C migrate from the back face to the display face is set to be different time from each other so that the slopes of each hysteresis Y, nY, M, nM, C, and nC become constant. For example, when the material design of the charged particles is done in such a manner that the drive voltage at least satisfies the demand for low power consumption, the voltages are |Vth(c)|≈7 (V), |Vth(m)|≈12 (V), |Vth(y)|≈28 (V), and the drive voltages are required to be set as |V3|=10 (V), |V2|=15 (V), and |V1|=30 (V).
In FIG. 18, the first (previous) screen is defined as white (W). When a voltage +V3 is applied, the electrophoretic particles of cyan (C) migrate to the display face side, so that cyan is displayed. When a voltage +V2 is applied, the electrophoretic particles of cyan (C) and magenta (M) migrate to the display face side, so that blue (B) is displayed. When a voltage +V1 is applied, the electrophoretic particles of cyan (C), magenta (M), and yellow (Y) migrate to the display face side, so that black (K) is displayed. Further, when the previous screen is white (W) and a minus voltage is applied, no color particle is on the display face side so that the screen remains as white (W).
In the meantime, when the previous screen is black (K) and a voltage −V3 is applied, the electrophoretic particles of cyan (C) migrate to the back face substrate side and the electrophoretic particles of magenta (M) and yellow (Y) remain on the display face side so that red (R) is displayed. When the previous screen is black (K) and a voltage −V2 is applied, the electrophoretic particles of cyan (C) and magenta (M) migrate to the back face substrate side and the electrophoretic particles of yellow (Y) remain on the display face side so that yellow (Y) is displayed. When the previous screen is black (K) and a voltage −V1 is applied, all the electrophoretic particles of cyan (C), magenta (M), and yellow (Y) migrate to the back face substrate side so that white (W) is displayed. Further, for displaying magenta (M), a voltage +V2 is applied and the displayed color is changed once from white (W) display to blue (B). Then, a voltage −V3 is applied to move the electrophoretic particles of cyan (C) to the back face so as to display magenta (M).
This operation can be expressed as follows. Assuming that the color density of the charged particles C in each pixel constituting a next screen to be updated is Rc, the color density of the charged particles M is Rm, and the color density of the charged particles Y is Ry, applied is a drive waveform that is constituted with:
a reset period where a reset voltage is applied to reset to a white or black base state;
a first voltage applying period where a first voltage V1 (or −V1) or/and 0 (V) voltage is applied to transit the base state into a first intermediate transition state where the color density of the charged particles C, M, and Y become Ry;
a second voltage applying period where a second voltage V2 (or −V2) or/and 0 (V) voltage is applied to transit the first intermediate transition state into a second intermediate transition state where the color density of the charged particles C and M become Rm while maintaining the color density of the charged particles Y as Ry; anda third voltage applying period where a third voltage V3 (or −V3) or/and 0 (V) voltage is applied to transit the base state into a third intermediate transition state where the color density of the charged particles C become Rc while maintaining the color density of the charged particles M and Y as Rm and Ry.
Methods for controlling particles that are not able to migrate or particles migrate again from the fixed positions in a typical electrophoretic display device are disclosed in Patent Documents 4 and 5.
Patent Document 4 describes a method which applies a first data input pulse or a second data input pulse whose data width is narrower or data strength is smaller than that of the first data input pulse in an image rewrite period in an electrophoretic display element of two-particle type with different polarities. As described, disclosed is a technique which improves the image quality through applying a prescribed voltage to the electrophoretic particles for the time required to migrate a prescribed distance between the electrodes and then applying a pulse for a shorter time or of a lower voltage than that between the electrodes to move the particles that were not able to migrate, the particles re-migrated from the fixed positions, and the like to the fixed position again.
Further, Patent Document 5 discloses a means for improving the screen retention capability (memory) through adding additional signals whose voltage changes gradually to the base potential after applying signals for displaying images on a display unit so as to suppress migration of the charged particles in the electrophoretic display element of two-particle type with different polarities. The above is done to apply an additional correction pulse for increasing the retention capability of the screen.
Patent Document 6 mentions that re-migration of the charged particles occurred during the transition from a screen update period to a retention period is caused due to an electric field by feed-through (to be described later). All the active matrix displays are subjected to an action called feed-through with which the voltage reaching the pixel electrodes changes from a corresponding data voltage input by a certain extent (normally 0.5 to 2.0 (V)). The feed-through effect is generated from scanning of the gate lines via a coupling electric circuit net between the gate lines which scan TFT and the pixel electrodes. That is, described in Patent Document 6 is as follows. The voltage actually applied to the pixel electrodes is shifted in a minus direction from the data voltage written to the pixel electrodes for the feed-through at the time of scanning Normally, for corresponding to the feed-through, the counter electrode is offset to the minus side from the reference potential (normally, ground GND) by a specific extent. Further, when scanning is stopped, there is an offset generated between the pixel electrodes and the counter electrode for the amount of the feed-through voltage.
As a countermeasure thereof, disclosed in Patent Document 6 is not to apply an unnecessary DC offset voltage through stopping scanning by fixing the scanning lines to a gate-off voltage and instantly switching the counter electrode voltage to GND from the feed-through voltage −Vfd by an analog switch in a non-wiring mode between a writing mode and power-off (retention).
Further, as the known technique described in Patent Document 7 and also employed in Patent Documents 5, 6, generally known is a technique with which the input pulse before turning off the power terminates at 0 (V). However, migration of the charged particles occurs also when the power is turned off from a screen update period where the display device is driven and an unnecessary electric field is applied to the elements. As will be described later in details, it is not possible to deal with such case with an additional correction pulse even if a correction pulse that terminates at 0 (V) is applied. Further, a mechanism with which the charged particles migrate when the power is turned off has not been analyzed. Furthermore, a waveform to be employed when multi-particles of different threshold values of Patent Documents 1 and 2 is not specifically disclosed therein.    Patent Document 1: Japanese Patent No. 4049202    Patent Document 2: Japanese Patent No. 4385438    Patent Document 3: Japanese Patent No. 4269605    Patent Document 4: Japanese Unexamined Patent Publication 2007-316594    Patent Document 5: Japanese Patent No. 4811510    Patent Document 6: Japanese Patent No. 4806634    Patent Document 7: Japanese Unexamined Patent Publication 2005-326883    Patent Document 8: Japanese Unexamined Patent Publication 2010-210806    Patent Document 9: Japanese Unexamined Patent Publication 2010-210660
There are following issues with the related techniques described above.
With the color electrophoretic display elements depicted in Patent Documents 1 and 2, the electric field sensitivity of the charged particles (C in the above-described case) with the lowest threshold voltage is extremely high so that the migration occurs even with the extent of about Vth(C)≈1 (V). Thus, after the screen update driving period (from a reset period to a third voltage applying period) ends and a voltage is continuously applied during a period from the point where the power supply voltage that is applied for driving the panel is turned off to a retention period, the charged particles migrated to the display face or the opposite face thereof leave from the substrate after update of the screen and mixed with W particles that are uncharged particles. Thereby, the colors may become unclear and variations in the color density are generated between pixels and recognized as unevenness, which result in deteriorating the display quality. Therefore, with the color electrophoretic display devices, it is necessary to control the layout of the charged particles after the screen update period.
In other words, when the power supplied for driving the panel is turned off with this driving method in the color electrophoretic display element using the charged particles which are mutually different in colors and threshold voltage for starting electrophoresis disclosed in Patent Documents 1 and 2, a weak voltage is continuously applied to the element by a difference in the discharge speeds between the pixel electrode voltage and the counter electrode voltage (to be described later). Thus, the large charged particles with the lowest threshold voltage migrate and the particles disposed once on the display face or the opposite face thereof leave from the substrate, so that there is an issue of deterioration in the retention capability (memory) with which the display image becomes unclear.
The techniques of Patent Documents 4 and 5 apply an additional correction pulse for improving the retention capability of the screen. However, migration of the charged particles occurs also when the power is turned off from a screen update period where the display device is driven and an unnecessary electric field is applied to the elements so that it is not possible to deal with such case with the additional correction pulse. Further, a mechanism with which the charged particles migrate when the power is turned off has not been analyzed. Furthermore, a waveform to be employed when multiple particles of different threshold values of Patent Documents 1 and 2 is not specifically disclosed therein.
In other words, while Patent Documents 4 and 5 disclose a technique for improving the retention capability of the screen by applying an additional correction pulse, the correction pulse is terminated at 0 (V) in the final sub-frame so that the unnecessary electric field applied to the element after the power is turned off cannot be eliminated. Therefore, the issue cannot be overcome substantially.
With the technique of Patent Document 6, it is not possible to perfectly follow the signal source even if the counter electrode voltage is changed instantly since the counter electrode is connected to the signal source via transfer resistance or sheet resistance of ITO (Indium Tin Oxide). Further, the pixel electrode is connected to the data line not via floating but via leak resistance of TFT, so that the pixel electrode voltage does not perfectly follow the change in the counter electrode voltage. Thus, the transit voltage of the counter electrode voltage and the transit voltage of the pixel electrode at the time of switching are different, and the DC offset generated due to the voltage difference cannot be eliminated completely. In particular, cyan particles (C) whose threshold voltage is small and the charged amount is large are to re-migrate because of the DC offset.
It is therefore an object of the present invention to overcome the above-described issues and to provide an image display device whose screen retention capability is improved by analyzing the mechanism with which the unnecessary electric field applied to the element after the power is turned off is generated and by devising the driving method and the like for enabling compensation thereof.